1. Field of the Invention
The present invention relates to two-port surface-acoustic-wave resonators (hereinafter referred to as "two-port SAW resonators"). In particular, it relates to a structural technology for suppressing spurious responses (unnecessary resonance) caused by higher-order vertical inharmonic modes in a two-port SAW resonator.
2. Description of Related Art
The basic electrode structure of a conventional two-port SAW resonator is disclosed in, e.g., U.S. Pat. No. 3,886,504, and a rotated Y-quartz plate in the case where an electrode film thickness is 1000 .ANG. or less is disclosed in Japanese Unexamined Patent Publication No. 61-251223. In addition, in Japanese Unexamined Patent Publication No. 61-230419, an example with three interdigital transducers (hereinafter referred to as "IDTs") is described.
In a two-port SAW resonator having three interdigital transducers, the central interdigital transducer is used as an input terminal, and on both sides thereof, the other interdigital transducers are disposed. Accordingly, the frequency of the two-port SAW resonator is stable for a change in the load of the output load circuit. The use of this type causes a temperature coefficient to change to zero in a so-called ST-cut two-port SAW resonator formed on an ST-cut X-transmission plate composed of single crystals of quartz. This is superior in terms of frequency stability similarly to that of a one-port type.
According to the above-described conventional art, when, in an ST-cut two-port SAW resonator having three interdigital transducers, an electrode film thickness is set at 1000 .ANG. or more so that the device size is reduced, resonance caused by a plurality of higher-order vertical inharmonic modes exists which is less than a main resonant frequency. There is a problem in that there is resonance caused by the plurality of higher-order vertical inharmonic modes, and a mode closest to the main resonant frequency causes a frequency jump to bring about inferior communication when the two-port SAW resonator is built into an oscillating circuit of a communication apparatus, because an oscillating-frequency-adjustment expansion coil allows frequencies to be close, whereby simultaneous excitation occurs.
In addition, despite the complete suppression of the higher-order vertical inharmonic modes, it is found that, when crossbus-bar conductors are disposed between the central interdigital transducer and the interdigital transducers on both sides thereof, two spurious responses having almost equivalent amplitudes are generated, depending on the crossbus-bar conductor structure.